I have two pairs of circular needles, I can not see what the size is that is printed on them, and I do have a knit check plate, how do I know what the size is when putting the needle in the holes, ? do the needles slide right in,? or do they stop at a certain point, ?

They should go in to the straight part of the shaft, after the tip. If they don't go past the tip, try the next size up. Some needles may be 'off size' a little and too small for one hole and loose in the next one; call it the looser size, it won't be off too much usually.